Alternating current hoist  control



9 w. R. WICKERHAM Re. 22, 923

\ ALTERNATING CURRENT HOIST CONTROL ori inai Filed Dec. 8, 194:; 2 Sheets-Sheet 1' 1' 1 Q WITNESSES: INVENTOR X M 52 7 William Mckerfiem.

ATTORNEY Sept. 30, 1947. w. R. WICKERHAM I ALTERNATING curmrzm' HOISI CONTROL I Original Filed Dec. 8, 194::

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BY a ATTORNEY Reissued Sept. 30, 1947 UNITED STATES PATENT. OFFICE ALTERNATIN G CURRENT HOIST'CONTROLZ.

William R; Wickerham, Swissvale, Pa., assignor to Westinghouse Electric Corporation, East. Pittsburgh, Pa., a corporation of Pennsylvania Original No. 2,386,580. dated October 9, 1945,

Serial No. 513,351, December 8, 1943: Application for reissue February 21, 1947, Serial No.

15 Claims.

My invention relates to alternating-current operated control systems for hoist motors on cranes, mine hoists and other hoisting and elevating devices and deals with subject matter related in some respects to that of my copending app1icationSerial No. 513,352, entitled Hoist control systems.

One of the objects of the present invention is to provide a hoist control system which, particu larly during-lowering operations, affords a high torque similar to that obtained by three-phase or so-called counter torque energization, while developing, when lowering, a reduced torque approaching the torque conditions of single-phase operation, when the master controller is set for low lowering speeds. In other words, the inven tion, in this aspect, aims at combining the advantages of multiphase and single-phase operation while avoidingor substantially reducing the disadvantages of these different types ofenergization.

A more specific object along these lines is to obtain a control which affords developing a high torque close to pull-out torque at high hoisting and high lowering speeds at a relatively low current inputbut reduces the torque to relatively slight values when setting the master controller for lowest lowering speeds.

Another object of the invention relates especially to hoist control systems which an alternating current hoist motor, such as a wound rotor motor, has an exterior secondary or rotor circuit equipped with resistors for counter torque braking andcontains control means for selectively eliminating parts or all of these resistors to provide several steps of progressive acceleration or deceleration. In the known systems of this type, just as many resistance steps are provided in the secondary circuit, requiring a corresponding number of secondary control relays, as the desired number of acceleration steps or corresponding control positions of the operator-actuated master controller. In contrast thereto, the invention aims also at providing a multi-step or wide-range acceleration control while reducing the number of secondary resistance steps and appertaining secondary contactor equipment below that heretofore necessary. For instance, the invention will achieve a control covering the ranges of, say, four to six customary control steps while using only two resistance steps in the exterior rotor circuit.

Still another object of my invention, also related to acceleration control by means of controlled resistors in the rotor circuit of an alterhating-current hoist motor, is to prevent or 'sub-' due the changes oftorque associated with an op.-

erator controlled:change of acceleration. That is, when advancing themaster controllepfrom.

one to another control point for changing the re.

r: sistance in the rotor circuit-the torque tends to change suddenly; andothe invention, in. .the pres'-.

ent aspect, aims at reducing or cushioning thisundesirable functioning.

It-is also an'object of my'inventionto permit in an alternating-current control system an opertion at alimited minimumspeedt-by. automatie cally controlling the torque exerted: b a1solenoid operated mechanical brake. Moreparticu larly, it is an object to limitthe sp'eed' of; any

' alternating-current. hoist motor when operating at light loadorno load "inthe lowering direction. Another object of the invention is tonlimit'th'e lowering speed even'th'ou'gh the system is-used inadvertently with heavy loadpresent. It isfllr thermore aimed at renderin'g. a separately con trollablebrake responsive to therope-rating. condi= tion of themotorso'that the brake force is greater.

ath'igh speed and automaticallyreducedas the speed decreases,

The invention'also aims at providing-a hoist control system in which'a single mastercontroller actuated by the operator is used for achieving the aims according to several or all of the above-- mentioned objects so asto resultin a control set up of simple design and operation with the-several functions performed. in apredetermined relation to one another and in dependence upon the instantaneous speed or torque conditionof the hoist motor.

These and other objects of the invention, will be apparent from: the following description-oi the embodiment shown in the drawings, in which:

Figure 1 represents the circuit'diagram oracomplete alternating-current hoist control-system in accordanceewith my invention,-

Fig. 2 is a sequence chart'relating to the: same exterior resistors .H and 12, BI and 82, 9| and, 92;

respectively. Therotorisdrives a shaft 6 which is coupled by a gear box GB with the hoist drum HD to be operated. A traveling nut l is controlled by the gear box so as to engage in one end position 'the movable contact member. 8 of a hoist limit 3 switch HS and in the other position the movable member 9 of a lower limit switch LS, depending upon the direction of rotation of the motor HM. A mechanical brake BR is connected with the drive shaft and is electrically energized by means of a magnet coil 6 I.

A hoist relay RI has its contacts III, 2 and 3I I controlled by a relay coil I I so that when the relay is energized the motor HM is energized for running in the hoisting direction. A lowering relay R2 having contacts II2, 2I2 and 3I2 controlled by a relay coil I2 serves to energize the motor HM for operation in the lowering direction.

A saturable reactor SR has its magnetizable core 5| provided with an alternating-current coil 52 and a premagnetizing control coil 53. Coil 52 is series arranged in the phase connection 3 of the hoist motor and is wound in two sections which are so located on core 5| so that virtually no transformer effect is produced on coil 53. A reactor control relay R3 is provided to short circuit the reactor SR by the closure of relay contact 3I3 when the appertaining relay coil I3 is energized. When contact 3I3 is closed, the reactor is ineffective so that the motor HM is energized for fullthree-phase operation and develops a corresponding three-phase torque characteristic. When contact 3I3 is open, the current supplied through the phase connection 3 depends as to magnitude on the effective reactance of coil 52. This reactance is controlled by the energization supplied to the reactor control coil 53, That is, if the current in coil 53 is low, the core 5| is magnetically unsaturated and hence the reactance of coil 52 at a correspondingly high value. On the other hand, when coil 53 is magnetized to such an extent as to saturate the magnet core 5I, the reactance of coil 52 is at a minimum. The range of variable reactance thus obtained is so chosen that the current in phase connection 3 is only slightly reduced when the reactor is saturated so that the motor HM is then energized for approximate three-phase operation, while when the reactor is unsaturated the effective reactance is so large that the current in phase connection 3 and the voltage across the respective primary are sufiiciently suppressed to operate the motor HM in approximate single phase operation. The means for effecting the just-mentioned reactor control will be described hereinafter.

The electromagnetic brake BR has its coil 6| connected to a rectifier bridge 62 through the contact I I5 of a brake relay R5 whose operating coil is denoted by I5. The rectifier bridge 62 is controlled by four relay contacts II4, 2I4, 3I4 and 4I4 appertaining to a relay R4. When the coil I4 of relay R4 is deenergized, contacts H4 and 3 I4 are closed and connect the rectifier bridge to the mains 2 and 3, Consequently, in this position and assuming that contact H5 of relay R5 is closed, the brake coil BI is energized from a substantially constant current source so that the :brake is fully released. When coil I4 of brake relay R4 is energized, contacts H4 and 3I4 are opened and contacts 2 I 4 and M4 closed. As a result, coil BI is now connected through a calibrating register 64 with two leads BI] and 90 of the rotor circuit and therefore energized by a voltage sufficiently low toreduce the braking effort at zero speed without releasing the brake entirely.

ince the voltage of this circuit depends on the speed of the motor, the, braking force applied by the brake is now dependent on the motor speed. That is, when the speed is low, a higher voltage is applied to coil BI, thus producing a lesser braking effect than when the motor speed is high and the secondary rotor voltage correspondingly reduced. The brake relay R4 has four additional contacts 5I4, H4, H4 and SM whose purpose will be discussed in a later place.

Two relays R6 and R1 are provided for controlling the resistance in the rotor circuit of the'hoist motor. Relay R6 has contacts I I6 and 2I6 which,

when closed, short-circuit all of the resistors in the rotor circuit. A further contact 3 I 5 serves an auxiliary purpose and is operated together with contacts H6 and 2I6 by a relay coil I6. Similarly, the rela R1 has contacts II! and 2I'I for shorting the resistors I2, 82 and 92 of the rotor circuit and an auxiliary contact 3I'I controlled by a relay coil II.

Two auxiliary relays R8 and R9 serve to control the operation of the hoist relay RI and R2 under certain operating conditions, as will be apparent hereinafter.

The system contains also two timing relays TI and T2 which are controlled by the operation of the secondary relays R3 and R7, Each of these time relays has a main magnetizing coil denoted by ID and 20, respectively, and an oppositely acting neutralizing coil denoted by I00 and 20D, respectively. The neutralizing coils have a smaller number of ampere turns than the main coils and serve to afford a definite time delay. Each relay is further provided with a short circuited winding consisting, for instance, of a copper sleeve or ring I30 or 230 for increasing the time constant. Relays of this type are known as such and do not form a feature of the invention proper, although the presence of such timing means is preferable in systems according to the invention for assuring a desired time limit between the subsequent operation of relays R6 and R1, or vice versa.

When energizing the supply circuit, the terminals G and D of the series connected neutralizing coils I00 and 200 are supplied with current so that these coils remain effective during the operation of the system. Due to the dimensioning of these neutralizing coils and since their effeet is in opposition to the active operation of the timing relays, the energization of the neutralizing coils does not effect the opening of the appertaining relay contacts. The main coils II! and 20 of the timing relay are connected at Eand F to a current source when the supply mains I, 2 and 3 are energized. However, the circuit of coil Iii extends over contact 3I5 of relay RE so that relay TI closes its contact III] only when the relay RE is energized, Similarly, the main coil 20 of time relay T2 is active only when contact 3I'l of relay R1 is closed and closes contact 2H] when deenergized.

In order to control the hoist motor HM for hoisting and lowering operation at different speeds, and for controlling the reactance of reactor SR in order to regulate the speed torque characteristic of the motor, a master controller MC is provided. This controller contains a plurality of contact fingers, such as those denoted by 43, which cooperate with a number of contact segments mounted on a common carrier or base 40. The two upper segments denoted by M and 42 serve to control the energization of the reactor control coil 53. To this end, coil 53 is connected to the master controller through a rectifier bridge 56 which is fed from points A and B or supply mains 3 and 2, respectively. The contact fingers appertaining to segments M and 42 are connected to taps of a resistor 55'which is arranged" in the direct-current circuit ofrectifi'er bridge 56- in series with control coil 53.

The brake control relay Rd has a coil I4 connected through conductors I BI and I62 between the master controller and a push button PB which in turn is connected to a conductor I32 leading to point C of main 2. The relay coil I4 is energized only when the push button PB is actuated and when at the same time the master controller establishes a connection of either conductor I6I or I62 with the conductor I33 leading to point A of main 3.

The operation of the system will be understood from the following description of the relay opertions, and the functions performed thereby, when the master controller MC is moved from the off position into the hoist and lowering positions denoted in Fig. 1 by numerals I through 8; and I through I, respectively. In the following description, reference is also had to the sequence chart of Fig. 2 which indicates the closure of the relay contacts in relation to the eight hoisting points and seven lowering points of the master controller.

According to Figs. 1 and 2, none of relays RI through R9 are energized when the master controller is in the off position. Hence contacts 3I6, 3II and 2 I8 are closed. As mentioned before, neutralizing coils Illil and 28!} of the time relays are energized. Main coils III and of time relays TI and T2 are also energized through E, II], 3I6, F and E, 2!], 351, F. Hence contacts III) and 2H] are open. The brake ER is deenergized because contact IE5 is open. The saturable reactor SR is connected in the motor circuit since contact 3I3 is open.

Starting from this oiT position, the following is a brief step by step description of a hoisting and a lowering operation.

HOISTING OPERATION Step 1 In this position of-the' master controller MC, resistor 55 is connected to rectifier set 55 by segment 4! of controller MC and has its full resistance in series to the reactor control coil 53. Hoisting relay BI is energized over B, II, HS, I38, MC, I33, A. Contacts III, 2 and 3 close and energize motor HM for hoisting.

Relay R5 is energized through C, I32, I5, I35, MC, I33, A, closes contact Il5. Hence, brake coil BI is connected to rectifier 62 and, R4 not being energized, is excited from the line to completely release the brake. Relays R3, R5 and R1 remain deenergized. Hence, SR is eifective and the total resistance in the rotor circuit is also effective. Since the premagnetization of SR by coil 53 extends over the full value of resistance 55, SR is not saturated and its reactance at a maximum. As a result, the motor is energized for approximate single-phase operation, that is, for hoisting at low speed and low torque. The speed-torque characteristic now effective is exemplified by the positive branch of curve C1 in Fig. 4.

Step 2 In this controller position, the uppermost sec- Steps 3 through 6 series resistor 55 is progressively diminished and tion of resistor 55 is shorted by segment 4| and 7 causes the magnetization of SR to increase and thus the reactance of coil 52 to decrease. There is no other change in the relay conditions as compared with those of Step 1.

the reactance of SR increased, thus increasing the current in phase connection 3. As a result,. the speed-torque relation approaches progressively a multi-phase characteristic. At step B-the resistance of resistor 55 is at a minimum or zero so that the full saturatin current is applied to control coil 53. In this condition, the magnet core 5| is saturated and hence the reactance of SR. at a minimum. The speed-torque characteristic obtained at step 6 is represented by curve. C2 in Fig. 4. This characteristic approaches that of a three-phase energization of the motor. the intermediate control steps, the torque characteristic then effective resumes a corresponding position intermediate curves C1 and C2.

Step 7 'deenergized by the opening of contact 3H. Contact 2H] closes with delay and thus prepares relay R6 for its subsequent operation. The re: moval of resistors I2, 82, e2 increases current input to the motor as well as the torque developed and tends to cause acceleration to curve C3. The torque does not change instantaneously to C3 since the increased current through SR results in'a temporary reduction in voltage on the motor terminal until such time as the accelerating current has disappeared. Thus, the saturable reactor produces also a cushioning effect resulting in gradual change in the speed torque conditions.

Step 8 The current and torque reactions described under '7 above are repeated with the result that speed-torque characteristics as shown by curve C4 are attained. More in detail, in addition to the adjustments obtained at step '7, the relay R6 is now energized over C,I32, 2H1, I8, I38, MC, I33, A, and shorts resistors 'II, 8|, ill at IIB'and 2I6I The opening of contact 3I5 effects the deenergization of coil It so that relay TI closes contact III] with delay. This brings relay R3 in over C, I32, IIII, I3, MC, I33, A. Contact 3I3is closed thereby shorting the reactor SR. As a result, the motor HM operates now in full three-phase energization with a correspondingly higher pull-out torque as represented by curve 05 in Fig. 4.-

LOWERING OPERATION To start the lowering operation the master lay R1 is energized through C, I32, I1, I31, M0,.

I34, LS, A and shorts resistors I2, 82, 92 at III" and 2H. Coil 20' of T2 is cut offat'3l'l so that 7 2H! closes with delay. Relay R is energized by way of A, LS, I34, MC, I36, I6, 2III, I32, C and shorts resistor 'II, 8|, 9| at IIS and 2I6. Hence the rotor circuit contains no exterior resistance.

The opening contacts 3 I 6 releases coil III of time relay TI so that contact III} is closed with delay. Coil I3 of R3 comes in over III] and closes contact 3 I3, thus shorting SR out of the primary circuit. Relay R8 comes in through C, I32, I8, 8I4, MC, I34, LS, A, and closes contact II8 thus energizing relay R9 through I32, I9, II8, MC, I34, LS, A. Relay R5 is energized over I32, I5, MC, I34, LS, A.

As a-result, the motor operates for high-speed lowering under power and the reactor SR is ineffective so that the motor develops full threephase torque.

Step 6 After passing from position 7 to position 6, the controller MC interrupts the circuit between I8 and I34 so that R8 falls off closing 2I8 and opening IIB. Coil I2 of R2 is likewise disconnected at MC and opens II2, 2I2 and 3I2. Relay R9 remains in because coil I9 is energized from I32 through the self-sealing contact 9, and energize RI through B, II, HS, I38, 2I8, II9, MC, I34, LS, A. Contacts III, 2, 3| I are closed for energizing the motor in opposition to the lowering load. Relay R5 remains in, maintaining II5 closed, while R6 and R1 are disconnected at MC so that resistors II, 8|, SI and I2, 82, 92 are effective in the rotor circuit for counter torque operation. The total resistance of 55 is inserted into the direct-current circuit of SR for reduced premagnetization and increased reactance. The torque characteristic now reached is similar to the negative branch of curve C: in Fig. 4.

Steps 5 through 1 During these subsequent steps, the relay and contact conditions of the system remain unchanged. Merely the resistance of 55 is stepwise dimensioned by segment 42 of MC to increase the magnetization of SR in order to change from approximately three-phase torque to approximate single-phase torque. At step I the resistance of 55 is at a minimum or zero and hence SR set for full saturation. During steps 6 and 1, the torque characteristic assumes positions between the negative branches of curves 0; and C1 in Fig. 4.

It is assumed in the above description of the function of the master controller that the push button PB remains in the illustrated open position in which the relay R4 is ineffective and hence connects the brake BR to the line for constant braking force. However, the operation of the master controller affects also the condition of readiness of the push button control. In this respect, it will be seen from Fig. 1 that the coil I4 of brake relay R4 is ready for energization only when the master controller is in the off position so that the push button PB is connected through coil I4, lead IGI and the master controller with lead I33 attached to point A of main 3. Due to the construction of the master controller, the push button PB remains ineffective during hoisting operations as well as during the high acceleration steps of lowering'operations. However, once pushed in the o position of the master controller, the push button PB can be kept effective over self-sealing contact 6 I 4 and lead I62 when the master controller is moved from oil into position 1 for slow lowering operation, but is cut off at MC when MC is moved farther into positions 2' to '7. When the master controller is moved out of the off. position before previously actuating the push button PB, the relay coil I4. cannot be energized.

The operation of relay R4 has the effect of changing the excitation of brake BR from the line to the secondary or rotor circuit of the hoist motor HM. As a result, the braking effect, when under push button control, is directly proportional to the motor lowering speed and is especially effective at high motor speeds. When controller MC is moved from off to lowering position 1 while button PB is kept pushed, the energization of relay R4 has the effect of actuating relay R2 through B, I2, I3I, II4, I40, MC, I33, A and applies lowering power to the motor primary. At the same time, contact 5I4 of R4 inserts resistor 54 into the premagnetizing circuit of the reactor RS- with the result of applying .a fixed degree of lowering torque at zero speed.

This torque falls to zero at synchronous speed, and this coupled with increasing brake torque serves to hold the motor at greatly reduced speed at lightor no load'as well as with normal load.

Reviewing the system and its operation, it will now be. apparent that a multi-step control of the speed torque characteristic of a hoist drive is obtained which, by virtue of the above-described saturable reactor feature, requires only a small number of resistance steps in the rotor circuit of the hoist motor. It will also be seen that the system affords obtaining high multiphase torques and at the same time offers the advantage of low torques at low speed lowering also by virtue of the above explained use of a saturable reactor. Furthermore, the system as described in the foregoing prevents sudden changes of torque when changing from one to another speed torque characteristic and this is.

also achieved with the aid of the saturable reactor.

This utilization of a single controllable impedance device for the just-mentioned different hoist control functions is one of the salient advantages of the illustrated embodiment and contributes essentially to attaining the above-mentioned functions by relatively simple and compact devices.

Another advantage of systems according to the invention is the fact that the regulation between three phase characteristic and approximate single phase (low speed low torque) characteristic is obtained Without changing the internal connections of the hoist motor. That is, a standard motor, having its primaries permanently connected with each other in delta or star connection, can be employed since the varia tion between these characteristics is achieved by externally connected control means, i. e., by varying the voltage distribution through a range which includes a substantially balanced three phase energization and a highly unbalanced or approximate single phase energization.

It will further be apparent that the reactor control, the control of the counter torque operation, and also the control of the push-buttonactuated braking operation are effected by means of a single master controller, an advantage also favorable to a simple design and operation of the system. By combining the push button with the mastercontroller, in particular, by placing the button in the handle or crank of the master controller, both are in effect capable of one-hand operation.

Since obvious various modifications on the basis .;of thei-above. disclosure and in accordance with the gist and scope of m invention areavailaa blelto those, skilledin the art, I Wish this specifii cation to beunderstood as illustrative and not in .a, limiting sense.

:I. claim as my invention: .;1. A control system-comprising an alternatingcurrent motor, multiphase circuit means for en- :ergizing: said; motor selectively, for operation in both directions, saturable reactance means forming part of said circuit means-and including pre- .magnetizing control windings, variable-energizing means connected to said windings for changing the motor energization between approximate one-phase and substantially multiphase operation, and operator-controlled selective step switch means connected to'said energizing means for controlling the latter in sequential steps.

2. A hoist control system comprising an alter- -nating-current hoist motor, three-phase circuit means for energizingsaid motor for hoisting and lowering operations, a saturable reactor forming part of said circuit meansand having a premagnetizing control coil for varying the reactanceof said reactor, a current source, and operator-controlled selective step switch means connecting said source withsaid coil for regulating said reactance-in sequential steps.

"ing premagnetizing control windings, variable energizing meansconnected to said windings for changing the motor energization between. ap-

proximate one-phase and multiphase operation and master control means connected with said multiphase control means. resistor control means and, energizing means for operating them sequentially, in a given relation to one another.

4. A control system comprising an alternatingcurrent motor, three-phase circuit means for energizing said motor, variable impedance means forming part of said circuit means and having an impedance range sufiicient for energizing said motor by substantiall single phase voltage and approximately balanced three-phase voltage, and selective control means under control by the operator and connected with said impedance means for progressively changing from said one to said other voltage.

5. A control system comprising an alternatingcurrent motor, three-phase energizing circuit means connected to said motor, a saturable reactor having a main coil series connected to said motor in one phase of said circuit means and a control winding for varying the reactance of said main coil over a range extending from approximate single-phase to approximately balanced three-phase energization of said motor, a direct current circuit connected to said control winding and comprising a stepped resistor for supplying variable energization to said control coil, and operator-actuated control means connected to said resistor for progressively changing the en.- ergization of said control coil in order to change the energization of said motor.

6. A hoist control system comprising an alternating-current hoist motor, three-phase energizing circuit means connected to said motor for energizing it for hoisting and lowering operations, a, saturable, reactor having av main coil series connected to said motor in one phase of said circuit meansanda control winding for varying the reactance of-said main coil, an energizing circuit connected to said control coil and containing astepped resistance device for varying the energization of said control coil to premagnetize said reactor between a minimum and maximum magnetization, and, a controller having a plurality of selective control positions and containing contact means connected with said circult means to control the latter and, further contact. meansconnected with said device for controlling itsresistance stepwise in accordance with the selected controller position-so that the reactance of saidreactor is stepwise decreased when changing the position of said controller towards high hoisting speed and increased when-chang- 1 ing toward high lowering speeds.

7. A. hoist control systemcomprising an alternating-current hoist motor, multiphase circuit means for; energizing, said motor for hoisting and lowering operations, a saturable reactor forming ,part of said circuit means, for varying the speed torque characteristic of said motor between predominant single-phase and multiphase opera tion, .a master controller for controlling said circuit 1means and said reactor in a given relation to each other,-and a. separately controllable power :-brake associated with said motor and connected I only when said circuit means and reactor are set to said master controller so as to be operable bysaid controller for zero speed and low speed lowering operation.

:8. A1hoist controlsystem comprising a hoist motor, circuit means for energizing said motor lforghoisting and lowering operations, variable impedance means. connected-with said motor for controlling its speed, a master controller for controlling said circuit'means and said impedance means in a given sequencasaid controller having an oil" position and a plurality of hoisting and lowering positions,a power brake associated with said motor and connected to said master controller so as to be effective in the off position and inefiective in the hoisting position of said controller, and separately actuable control means operable by the operator for releasing said brake when said master controller is in said off position.

9. A hoist control system comprising an alternating-current hoist motor having a primary energizing circuit and a secondary rotor circuit, a friction brake connected with said motor, electric means for energizing said brake normally connected to said primary circuit, a master controller having an off position and a plurality of hoisting and lowering positions and being connected with saidprimary and secondary circuitsfor operating said motor at different hoisting and lowering speeds, and separately actuated operator-controlled contact means for disconnecting said brake energizing means from said primary circuit and connecting it instead to said secondary circuit for speed-responsive operation, said contact means being electrically connected with said master controller so as to be ineifective in hoisting and high-speed lowering positions of said controller.

10. A hoist control system comp-rising an alternating-current hoist motor, energizing circuit means connected to said motor, a controller having an off position and hoisting and lowering positions and being connected with said circuit means to control said motor in accordance with 11 said positions, a mechanical brake connected with said motor, current supply means for providing substantially constant energization of said brake, speed-responsive current supply means for energizing said brake in accordance with the motor speed, changeover relay interposed between said brake and said different supply means so as to normally connect said brake to said constant supply means, an operator-actuated separate switch for controlling said relay to change said brake over for energization by said speed-responsive means, said switch being connected with said controller so as to be effective for starting a changeover only in said off position, said relay having a self-sealing contact connected with said controller so as to permit maintaining said switch efiective only over a limited number of low-speed lowering positions of said'controller once said switch has been actuated in a proceeding off position, whereby the system afiords stopping said motor at no .load and low-load lowering.

11. A motor control system, comprising a multi-phase alternating-current motor, multiphase supply means connected to said motor for supplying alternating-current energization, control-, lable impedance means disposed in, a lesser num ber of phases of said supply means for varying the phase distribution of said energization, a control circuit associated with said impedance means for controlling the impedance condition of the latter in response to changes of a control voltage, voltage supply means connected with said circuit for applying said control voltage thereto and having operator-actuable selective contact means for selectively varying said control voltage.

12. A motor control system, comprising a mul tiphase alternating current motor, multiphase supply means connected to'said motor for supplying alternating-current energization,contro1- lable impedance means disposed in a lesser number of phases of said supply means for varying the phase distribution of said energization, control means forming'part of said impedance means for controlling the impedance condition of the latter means, and voltage supply means associated with said motor and connected to said controlmeans for providing a 'control voltage sub- '12 stantially in accordance with the speed of said motor.

13. A hoist control system comprising, in'combination, an alternating circuit hoist motor having standard connected primary windings, a multiphase circuit connected to said primaries and containing variable reactor means associated with a lesser number of phases for varying the voltages of said primaries relative to each other through a range including a substantially balanced multiphase voltage distribution and a greatly unbalanced voltage distribution to obtain corresponding low speed high torque and low speed low torque characteristics respectively, and anv operator actuable controller for varying the reactance of said reactor means through selective intermediate stages corresponding to speed torque characteristics intermediate the aforesaid characteristics. I

14. A hoist control system comprising an alternating-current hoist motor, three-phase current supply means connected to said motor, a variable impedance device disposed ina'lesser number of phases of said supply means and having animsubstantially balanced three-phase energizationat hoisting operation of said motor.

15. A hoist control system comprising an alternating current hoist motor, three-phase current.

supply means connected to said motor, saturable reactor means associated with a lesser number of phases of said supply means, control windings for varying the reactance of said reactor means, circuit control means associated with said supply means for operating said motor for hoisting and.

lowering, and control means connected to said control windings for increasing said reactance so as to provide unbalanced energization to said motor at overhauling lowering. operation.

WILLIAM R. WICKERHAM. 

